This invention relates generally to pulse circuits and more particularly to systems for preventing the false triggering of alarm systems or the like by extraneous signals.
Alarm systems provide alarm signals that have a characteristic frequency within a predetermined range. For example, sonic intrusion devices such as the type described in U.S. Pat. No. 3,754,222 provide periodic signals having a frequency in the range of 3-12 Hertz to indicate human intrusion. Combined with the intrusion signal is extraneous information consisting of low frequency signals caused by environmental changes, and infrequently occurring high frequency bursts of short duration pulses originating from a variety of sources such as atmospheric disturbances, including lightening, and radio interference. Such extraneous signals can cause false triggering of the alarm system; and it is desirable to provide a system that is responsive to the alarm signal, but does not respond to the extraneous signals.
Systems for reducing the probability of falsing (i.e., false triggering) of alarm systems are well known. Various approaches may be taken to reduce the probability of falsing. One such approach is a variable threshold approach wherein the sensitivity of the alarm system is adjusted to a predetermined level in order to make the system responsive only to signals exceeding that predetermined level. The predetermined level may be either manually adjusted or automatically adjusted by means of a feedback loop similar to an automatic gain control circuit. Another approach is to provide timing circuitry that renders the circuit nonresponsive to signals shorter than a predetermined time duration, and a third approach utilizes multiple sensors and differential circuitry for rejecting common mode signals impinging on all the sensors while remaining responsive to localized signals impinging on less than all the sensors.
Whereas these approaches reduce the falsing of alarm systems, the threshold systems necessitate a compromise between sensitivity and falsing performance because if the threshold is set high enough to eliminate most of the falsing, a signal indicating a genuine alarm condition may not be detected. Even automatically adjusted threshold systems have serious drawbacks because the sensitivity is substantially reduced by the feedback loop in noisy environments, and the system remains subject to falsing in a quiet environment where a single high amplitude extraneous pulse may be sufficient to falsely trigger the system.
The time delay systems provide some improvement in performance over the threshold systems, however, they suffer from the disadvantage that a relatively short alarm indicating signal may be missed if its duration is shorter than the time delay of the falsing protection circuit. Common mode rejection is multiple sensor type systems provides no falsing protection to locally confined extraneous signals. Furthermore, a genuine alarm condition applied to all sensors would not trigger the alarm because of the common mode rejection characteristics of the circuit.